182 ODOXTORNITHES. . 



Behind the axis, or where the vertical motion prevails, we find in 

 modem birds no exception to the saddle articulation of the vertebrae in the 

 whole cervical series. 



In the dorsal vertebrae, this cause would be less efficient, since the 

 ribs and neural spines tend to restrict vertical motion, and hence to arrest 

 this modification. This region, then, as might be expected, offers strong 

 confirmatory evidence of the correctness of the above explanation; for 

 here occur, among existing birds, the only true exceptions known in the 

 presacral series to the characteristic saddle-shaped articulation. In Strigops 

 and a few other land birds ; in the Penguins, the Terns, and some other 

 aquatic birds, one or more vertebras in the dorsal region are without the 

 saddle-shaped articulation, and are either opisthoccelian, or imperfectly 

 biconcave. In such instances, we can usually, if not always, detect 

 evidence of an arrest of vertical flexure. This may lock together the 

 posterior dorsals by their neural spines, as in Strigops, leaving the power 

 of lateral flexure ; or several vertebrae may be coossified, as in Accipiter, 

 and some other Baptores, in which a stiff back is a positive advantage. 



In the coossified sacral series of many birds, one or more of the 

 anterior vertebrae have the saddle-shaped articulation. This, however, is 

 no valid objection to the above explanation, since these vertebrae are really 

 dorsals, and have gradually coalesced with the true sacral vertebrae. 



In the caudal vertebrae of recent birds we have, in a measure, the 

 original biconcave structure preserved, for here the motion in every direc- 

 tion was much restricted. The caudal vertebrae of these birds, even in the 

 most aberrant forms, are essentially the same, and in the fossil species the 

 articulations at least appear to follow the general rule. In Pavo and 

 Geoccocyx, the caudal vertebra? exhibit a tendency to a proccelian union. 

 Some other forms also show unimportant modifications of the normal type 

 of caudal articulation, but nothing to suggest a real objection to the 

 explanation now proposed of the origin of the saddle-shaped vertebrae, 

 characteristic of Birds. 



In bringing together the above facts, and others suggested bv them, 

 the classification and development of the various forms of vertebrae appear 

 to be somewhat as follows : 



